Metalloenzymes catalyze a wide variety of oxidative and reductive reactions. Electron paramagnetic resonance (EPR) spectroscopy has been widely used to directly probe the paramagnetic active site of metalloenzymes. The specific aims of this project are to, first, develop new methods using EPR spectroscopy for observing the paramagnetic centers associated with photosynthetic water oxidation and, second, to use these methods, in addition to those already available, to characterize the identity and structure of the components at the active site of water oxidation and to follow the transfer of electrons from water through Photosystem II. The overall goal of this work is to unravel the mechanism whereby plants and algae carry out the important yet difficult oxidation of water. In order to characterize the catalytic site for water oxidation, the binding and/or chemical reaction of small molecules, known to interact with the active site, will be monitored. In addition, the flow of electrons in Photosystem II will be followed when paramagnetic intermediates are formed at low temperature and allowed to decay by incremental warming. To facilitate the analysis of EPR spectra from a cluster of metal ions, as expected in this system, theoretical work will be undertaken to allow EPR spectra from sites with interacting metal ions to be calculated. The attainment of these specific aims will set the groundwork for a detailed investigation of the mechanism of photosynthetic water oxidation, a process central to photosynthetic carbon fixation. Methods developed in this work may also be applied to the study of other metalloenzymes, particularly those containing metal ion clusters.